Function expressions are a way to create functions in JavaScript by assigning them to variables. They are used when we need to create a function on the fly, or when we need to pass a function as an argument to another function. Function expressions are used instead of normal functions when we need to create a function that is used in one particular line of code and will not be reused later. Here is an example in JavaScript:

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// Normal function
function sum(a, b) {
  return a + b;
}

// Function expression
const sum = function(a, b) {
  return a + b;
}

In this example, sum is created both as a normal function and as a function expression. Useful resources to understand more about that are:

• MDN Web Docs: Function Expressions
• W3Schools: Function Expressions

A function is a block of organized, reusable code that is used to perform a single, related action. Functions provide better modularity for your application and a high degree of code reusing. In JavaScript, functions are first-class objects, because they can have properties and methods just like any other object. There are several ways to create a function:

• Function Declaration
• Function Expression
• Arrow Functions
• Function Constructors

Below is an example of a function declaration in JavaScript:

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// example of a function declaration
function sayHello(){
  console.log('Hello!');
}

// call the function
sayHello();

This is an example of what the output looks like when this code is run:

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Hello!

A let statement in JavaScript declares a variable with block scope. It's similar to a variable declared with var, however let allows you to define a variable with a limited scope at the block, statement, or expression level. For example:

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function checkScope() {
  let i = "function scope";
  if (true) {
    let i = "block scope";
    console.log("Block scope i is: ", i);
  }
  console.log("Function scope i is: ", i);
  return i;
}

The output of the code above will be:

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Block scope i is: block scope
Function scope i is: function scope

More information on the let keyword can be found on Mozilla Developer Network.

An arrow function expression is a syntactically compact alternative to a regular function expression, although without its own bindings to the this, arguments, super, or new.target.

Syntax:

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(param1, param2, …, paramN) => { statements } 
(param1, param2, …, paramN) => expression
// equivalent to: => { return expression; }

// Parentheses are optional when there's only one parameter name:
(singleParam) => { statements }
singleParam => { statements }

// The parameter list for a function with no parameters should be written with a pair of parentheses
() => { statements }

More info: MDN Arrow Functions

In JavaScript, an arrow function (also called a fat arrow function) is a concise way to write a function expression. It has a more concise syntax than a standard function expression and does not bind its own this, arguments, super, or new.target.

Syntax:

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(param1, param2, …, paramN) => { statements }
(param1, param2, …, paramN) => expression
// equivalent to: (param1, param2, …, paramN) => { return expression; }

// Parentheses are optional when there's only one parameter name:
(singleParam) => { statements }
singleParam => { statements }

// A function with no parameters requires parentheses:
() => { statements }

Source: MDN

You can initialize a variable in an if else expression by declaring the variable before the statement, and setting it equal to the desired value depending on the result of the condition.

For example, in JavaScript:

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let result;
if (x > 10) {
  result = x - 10;
} else {
  result = x + 10;
}

If you want to replace one item with another, one of the simplest ways to do this is by using the replace() function in any programming language. For example, in JavaScript you can write:

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let myString = 'Hello, world!';
let newString = myString.replace('world', 'Athena');
//newString = 'Hello, Athena!'

The replace() function will take two arguments: the first is whatever text string you want to search/replace, and the second is the replacement string. In the example above, the replace() function searches for the string world and replaces it with the string Athena within themyString variable.

For more information about the replace() function, check out this MDN web docs page.

In JavaScript, you could use a ternary operator to create a prompt with different spellings. A ternary operator is an expression that takes three operands and evaluates the expression depending on a boolean evaluation of the first operand.

In the below example, the first operand is a prompt used to ask the user to select whether they have either "apple" or "Apples". The second operand is the value on the left side of the colon, which is a prompt with the spelling "apple". The third operand is the value on the right side of the colon, which is a prompt with the spelling "Apples".

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var selection = prompt("Do you have Apple or Apples?") == "apple"? "apple": "Apples";

The example is taken from this article.

An if statement is a type of control flow statement that allows you to execute certain code if a certain condition is met. In most programming languages, an if statement has the following basic syntax:

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if (condition) {
    // Statement(s) will execute if the condition is true
}

Here, condition can be any expression that evaluates to a boolean value - true or false.

For more information on if statements, please refer to this tutorial from W3 Schools.

In some programming languages, two exclamation marks before a variable mean "not". It is an operator used to negate the value of a boolean expression i.e. it will evalute the expression to the opposite of its normal truth value.

For example, in JavaScript:

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let a = true;
let b = !!a;
console.log(b); // --> true

In the code example above, the first let statement sets a value of true. Then, the two exclamation marks before the a negates the value, and b is set as true.